Abstract

RASCON-HEIMPEL, Mario Arturo; VALENCIA-MORENO, Martín; OCHOA-LANDIN, Lucas  and  CALMUS, Thierry. Geology and geochemistry of Los Alisos porphyry copper, La Caridad mining district, Sonora, Mexico. Rev. mex. cienc. geol [online]. 2012, vol.29, n.2, pp.355-383. ISSN 2007-2902.

Los Alisos prospect is a porphyry copper deposit located in northeastern Sonora, at about 10 km northeast of La Caridad mine. It is a relatively small deposit, characterized by an intense hypogene oxidation zone exposed over 20 km². According to the new U-Pb zircon dates reported in this work, the oldest rocks outcropping in this region consist of andesitic volcanic flows of Late Cretaceous age dated at 73.6 ± 1.0 Ma, which were intruded during the Paleocene by a 60.2 ± 0.8 Ma old quartz-feldspar porphyry stock. These rocks were unconformably covered by Eocene-Oligocene rhyolite lava flows and tuffs. Locally, these rocks were in turn covered by a sequence of conglomerate and sandstone with basaltic lava flows, which regionally correlates with the Miocene Baúcarit Formation. The geochemical and isotopic composition of the quartz-feldspar porphyry is characterized by a light rare earth enriched pattern (LREE), coupled with Sr and eNd initial values of 0.7067 and -5.5, respectively. This is consistent with compositions of the intrusive rocks of the Laramide belt that were emplaced in southwestern U.S.A. and northwestern Mexico, in the region underlain by the North American Proterozoic basement. The study area is structurally complex, especially due to the effects of the regional extension associated with the development of the Basin and Range province. As a result of this extensional event, the deposit of Los Alisos occurs in an uplifted block, which is bounded by two NNW-oriented normal faults that juxtapose the Laramide volcanic sequence with Eocene-Oligocene rhyolitic lavas and Miocene rocks of the Báucarit Formation. Unlike the large mineralized block of La Caridad, where exhumation exposed the hypogene sulfide zone to the action of meteoric water, generating an outstanding concentration of copper in supergene enrichment zones, at Los Alisos the main ore body is not exposed but may be located at some uncertain depth.

Keywords : porphyry copper; Laramide magmatism; Los Alisos; La Caridad; Sonora; Mexico.

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